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USB Power Design Issue - Any one any ideas?!?

Hello,

I recently bought a new head unit for my car which has a USB port on it, for the sole purpose of being able to read MP3's. So I foolishly bought a 60Gb hard drive to go with it.

Set it all up, and it turns out the Headunit can not provide enough power to spin the Harddrive up so......

What I want is more than just a simple 7805 regulator. Being a harddrive I want it to be shut down properly each time I turn the head unit off, because I dont want bad data/ head crashes onto my brand new drive!

So, what I was thinking was some sort of switched 5v supply, switched on by the 5v that the Head unit provides on the USB connection. The limit being this 5V can only supply 500mA Max, so ideally I would only want to draw around 250mA if possible.

The two options I have thought of were, have a 7805 connected permenatly, and then use the 5V from the USB to "Switch" the regulated

5v on to the hard drive. (Thought about this method, was not too keen on having the 7805 live all the time)

Or the second method, use the 5V from the USB to turn on the 12v to the

7805 regulator. However I'm totally stumped with this one., Thought about a relay perhaps? Wanted to avoid this one though as I dont possess any and thought maybe a relay would draw too much current from the head unit?

Thanks in Advance

Ben

Reply to
benjamin.jennings
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It won't. I would use a 5V relay, and you must put a 6.2V zener diode across the coil, as the switching mechanism in the head end may not be designed for inductive loads.

Something like 619-3007 from

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.

5V coil, >1A.

On the 7805, as protection, you want before it a 1A fuse, 1 ohm 3W resistor, then a 1n4007 diode, leading up to a 1000uF 25V capacitor, with a 100nf capacitor across it. Then the 7805, with a similar set of capacitors on the output.

Also, put the regulator next to the hard drive, and feed it a switched

12V line, rather than running 5V all the way to the drive.
Reply to
Ian Stirling

Get a 12v switcher, that has a 'shutdown' input. Connect this to the USB

5v. Play it carefully, regarding trapping and suppression on all the lines (the car, is a _nasty_ enviroment for spikes etc.).

Best Wishes

Reply to
Roger Hamlett

Thank you all for the suggestions, one other idea thats hit me, is to perhaps use the 5V to drive the base of a trransistor. Connecting the emitter and collector to the 12v supply rail and ground, and drive the

7805 that way?!?
Reply to
benjamin.jennings

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